Firm profile

Westfield Capital Management Company, L.P.

BOSTON, MACRD 146990Registration Status: ApprovedEffective since June 30, 2008SEC # 801-69413Latest ADV July 1, 2026

Westfield Capital Management Company, L.P. is an SEC registered investment adviser organized as a limited partnership with a primary office location in Boston, MA. Its latest Form ADV filing, dated July 1, 2026, reports $29.8B in regulatory assets under management, 63 employees, 1 office, and 9,391 accounts.

Assets Under Management

$29.8B

Employees

Offices

1 office

Total Clients

9,391

Associated firm names

1 name listed

WESTFIELD CAPITAL MANAGEMENT COMPANY, L.P.

Leadership

6 roles identified

CEOWilliam A. Muggia

CCOKathryn A. Kearney

CFOKathryn A. Kearney

Advisory services offered

4 types reported

Portfolio management for businesses

Portfolio management for individuals and small businesses

Portfolio management for investment companies

Fee models offered

2 types reported

Asset based fees

Performance-based fees

Offices & Contact

Primary office

BOSTON, Massachusetts, United States

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Primary office regionOther reported regions

Shading shows states and countries with reported offices. Cities are listed below; shading does not indicate service coverage.

View locations

Massachusetts

BOSTONPrimary office

1 office records listed · 1 total offices reported. Counts can use different reporting dates. SEC filers may list only their largest 25 additional offices.

Contact and digital presence

ADV profile links

Phone(617) 428-7100 WebsiteVisit website LinkedInView profile

Regulatory Assets Under Management Breakdown

Total regulatory assets under management$29.8Bas of July 1, 2026

Discretionary100.0%

$29.8B

100% of total RAUM100.0%

Non-discretionary0.0%

0% of total RAUM0.0%

All regulatory assets under management are discretionary.

Client Coverage Demographics

Top client segmentIndividuals represent 89.4% of total clients.

Top client categories% of clientsClients

Individuals

89.4%

8,398

HNWI

5.5%

512

Pension Plans

1.9%

177

Charitable

1.5%

138

Corporations

1.0%

Additional categories (<0.1%)

Other Advisers<5 clients

Categories with <5 clients

Other investment advisers<5 clients

Assets under management Growth by Quarter

Quarterly points use the latest ADV filing or SEC roster snapshot available in each quarter.

Total AUM$29.8B

Total Accounts9,391

10Y AUM Change+$18.1B (+153.6%)

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Employee Growth by Quarter

Quarterly points use the latest ADV filing or SEC roster snapshot available in each quarter.

Total Employees63

Clients per Employee149:1

10Y Employee Change-1 (-1.6%)

Historical AUM and employee figures reflect available SEC Form ADV filings under this CRD. Filing gaps may reflect registration status changes, dormant periods, state registration periods, firm restructuring, or other reporting changes. Rialytix does not interpolate missing periods.

Reported ownership ranges

Highlighted bands show the percentage range reported in Form ADV.

0%25%50%75%100%

WMS MANAGEMENT LLCDirect owner · Schedule AInterest in WESTFIELD CAPITAL MANAGEMENT COMPANY, L.P.

75%+

MG AM HOLDING, INC.Direct owner · Schedule AInterest in WESTFIELD CAPITAL MANAGEMENT COMPANY, L.P.

10% to < 25%

WMS GENERAL PARTNER LLCDirect owner · Schedule AInterest in WESTFIELD CAPITAL MANAGEMENT COMPANY, L.P.

< 5%

KATHRYN ANN KEARNEYDirect owner · Schedule AInterest in WESTFIELD CAPITAL MANAGEMENT COMPANY, L.P.

< 5%

WMS MANAGEMENT LLCIndirect owner · Schedule BInterest in WMS GENERAL PARTNER LLC

75%+

WILLIAM ALBERT MUGGIAIndirect owner · Schedule BInterest in WMS MANAGEMENT LLC

50% to < 75%

Bars represent the reported range at that individual ownership step, not an estimated percentage of the advisory firm. Indirect relationships may refer to intermediate entities rather than directly to Westfield Capital Management Company, L.P..

Reported ownership and control details

Form ADV · Filing date 2026-07-01 · Schedules A and B

17 entries

#	Owner / person & role	Interest in entity	Reported interest	Control person	Title / status acquired	Source
1	WMS GENERAL PARTNER LLCGENERAL PARTNERDomestic entity	WESTFIELD CAPITAL MANAGEMENT COMPANY, L.P.	Less than 5%Code NA	Yes	GENERAL PARTNER06/2008	Schedule APDF page 36
2	WMS MANAGEMENT LLCLIMITED PARTNERDomestic entity	WESTFIELD CAPITAL MANAGEMENT COMPANY, L.P.	75% or moreCode E	Yes	LIMITED PARTNER06/2008	Schedule APDF page 36
3	WILLIAM ALBERT MUGGIACHIEF EXECUTIVE OFFICER AND CHIEF INVESTMENT OFFICERIndividual	WESTFIELD CAPITAL MANAGEMENT COMPANY, L.P.	Less than 5%Code NA	Yes	CHIEF EXECUTIVE OFFICER AND CHIEF INVESTMENT OFFICER11/2001	Schedule APDF page 36
4	KATHRYN ANN KEARNEYPARTNER, CHIEF FINANCIAL OFFICER AND CHIEF COMPLIANCE OFFICERIndividual	WESTFIELD CAPITAL MANAGEMENT COMPANY, L.P.	Less than 5%Code NA	Yes	PARTNER, CHIEF FINANCIAL OFFICER AND CHIEF COMPLIANCE OFFICER12/2012	Schedule APDF page 36
5	JOHN MORGEN MONTGOMERYCHIEF OPERATING OFFICERIndividual	WESTFIELD CAPITAL MANAGEMENT COMPANY, L.P.	Less than 5%Code NA	Yes	CHIEF OPERATING OFFICER01/2015	Schedule APDF page 36
6	RICHARD DAILYONG LEECHIEF INVESTMENT OFFICERIndividual	WESTFIELD CAPITAL MANAGEMENT COMPANY, L.P.	Less than 5%Code NA	Yes	CHIEF INVESTMENT OFFICER01/2016	Schedule APDF page 36
7	MG AM HOLDING, INC.LIMITED PARTNERDomestic entity	WESTFIELD CAPITAL MANAGEMENT COMPANY, L.P.	10% to less than 25%Code B	No	LIMITED PARTNER04/2025	Schedule APDF page 36
8	WMS MANAGEMENT LLCSOLE MEMBERDomestic entity	WMS GENERAL PARTNER LLC	75% or moreCode E	Yes	SOLE MEMBER06/2008	Schedule BPDF page 37
9	WILLIAM ALBERT MUGGIAMEMBER OF MANAGEMENT COMMITTEEIndividual	WMS MANAGEMENT LLC	50% to less than 75%Code D	Yes	MEMBER OF MANAGEMENT COMMITTEE06/2008	Schedule BPDF page 37
10	ROBERT TIZOC FLORESMEMBER OF MANAGEMENT COMMITTEEIndividual	WMS MANAGEMENT LLC	Percentage not specifiedCode F	Yes	MEMBER OF MANAGEMENT COMMITTEE01/2013	Schedule BPDF page 37
11	RICHARD DAILYONG LEEMEMBER OF MANAGEMENT COMMITTEEIndividual	WMS MANAGEMENT LLC	Percentage not specifiedCode F	Yes	MEMBER OF MANAGEMENT COMMITTEE01/2013	Schedule BPDF page 37
12	JOHN MORGEN MONTGOMERYMEMBER OF MANAGEMENT COMMITTEEIndividual	WMS MANAGEMENT LLC	Percentage not specifiedCode F	Yes	MEMBER OF MANAGEMENT COMMITTEE01/2013	Schedule BPDF page 37
13	KATHRYN ANN KEARNEYMEMBER OF MANAGEMENT COMMITTEEIndividual	WMS MANAGEMENT LLC	Percentage not specifiedCode F	Yes	MEMBER OF MANAGEMENT COMMITTEE01/2021	Schedule BPDF page 37
14	JENNY ANN MULLERMEMBER OF MANAGEMENT COMMITTEEIndividual	WMS MANAGEMENT LLC	Percentage not specifiedCode F	Yes	MEMBER OF MANAGEMENT COMMITTEE01/2021	Schedule BPDF page 37
15	MATTHEW ROBERT RENNAMEMBER OF MANAGEMENT COMMITTEEIndividual	WMS MANAGEMENT LLC	Percentage not specifiedCode F	Yes	MEMBER OF MANAGEMENT COMMITTEE01/2021	Schedule BPDF page 37
16	JUSTIN MATTHEW MOSCARDELLIMEMBER OF MANAGEMENT COMMITTEEIndividual	WMS MANAGEMENT LLC	Percentage not specifiedCode F	Yes	MEMBER OF MANAGEMENT COMMITTEE01/2024	Schedule BPDF page 37
17	MONEX GROUP, INC.SHAREHOLDER OF MG AM HOLDING, INC.Foreign entity	MG AM HOLDING, INC.	Percentage not specifiedCode F	No	SHAREHOLDER OF MG AM HOLDING, INC.04/2025	Schedule BPDF page 37

Less than 5% may include zero. Code F identifies a role without specifying a percentage. Reported interests may overlap and may not include every owner. Total indirect ownership is not calculated.

Compliance & Disclosures

Filing dated 7/1/2026 2:28:38 PM

Responses were selected by the RIA in its filing. Questions are shortened for readability; “you” refers to the reporting firm, and some questions also cover advisory affiliates.

RIA or supervised-person involvementThe RIA reported that none of the events below involve the firm or its supervised persons.RIA responseNo

AFelonies2 No

In the past ten years, have you or any advisory affiliate:

A1Been convicted of or pleaded guilty or no contest to a felony?No
A2Been charged with a felony?No

BMisdemeanors2 No

In the past ten years, have you or any advisory affiliate:

B1Been convicted of or pleaded guilty or no contest to a misdemeanor involving investments, an investment-related business, fraud, false statements or omissions, wrongful taking of property, bribery, perjury, forgery, counterfeiting, or extortion?No
B2Been charged with a misdemeanor of the kind listed above?No

CSEC or CFTC actions5 No

Has the SEC or CFTC ever:

C1Found you or an advisory affiliate made a false statement or omission?No
C2Found you or an advisory affiliate violated SEC or CFTC regulations or statutes?No
C3Found you or an advisory affiliate caused an investment-related business to have its authorization to do business denied, suspended, revoked, or restricted?No
C4Entered an order against you or an advisory affiliate in connection with investment-related activity?No
C5Imposed a civil money penalty on you or an advisory affiliate, or ordered you or an advisory affiliate to cease and desist from any activity?No

DOther regulatory actions5 No

Has any other federal regulatory agency, state regulatory agency, or foreign financial regulatory authority ever:

D1Found you or an advisory affiliate made a false statement or omission, or was dishonest, unfair, or unethical?No
D2Found you or an advisory affiliate violated investment-related regulations or statutes?No
D3Found you or an advisory affiliate caused an investment-related business to have its authorization to do business denied, suspended, revoked, or restricted?No
D4In the past ten years, entered an order against you or an advisory affiliate in connection with investment-related activity?No
D5Denied, suspended, or revoked your or an advisory affiliate’s registration or license, or otherwise prevented association with an investment-related business or restricted activities?No

ESelf-regulatory organization actions4 No

Has a self-regulatory organization or commodities exchange ever:

E1Found you or an advisory affiliate made a false statement or omission?No
E2Found you or an advisory affiliate violated its rules, other than a violation designated as a minor rule violation under an SEC-approved plan?No
E3Found you or an advisory affiliate caused an investment-related business to have its authorization to do business denied, suspended, revoked, or restricted?No
E4Disciplined you or an advisory affiliate by expulsion, suspension, barring association with other members, or otherwise restricting activities?No

FProfessional authorizations1 No

FHas an authorization to act as an attorney, accountant, or federal contractor granted to you or an advisory affiliate ever been revoked or suspended?No

GPending regulatory proceedings1 No

GAre you or any advisory affiliate now the subject of a regulatory proceeding that could result in a “Yes” answer to any part of C, D, or E?No

HCivil judicial actions and proceedings4 No

(1) Court actions

Has a domestic or foreign court:

H(1)(a)In the past ten years, enjoined you or an advisory affiliate in connection with investment-related activity?No
H(1)(b)Found that you or an advisory affiliate violated investment-related statutes or regulations?No
H(1)(c)Dismissed, pursuant to a settlement agreement, an investment-related civil action brought against you or an advisory affiliate by a state or foreign financial regulatory authority?No

(2) Pending civil proceedings

H(2)Are you or any advisory affiliate now the subject of a civil proceeding that could result in a “Yes” answer to any part of H(1)?No

* Metrics are based on the latest available data, including Form ADV filings, brochures, and relationship summaries pulled from the SEC IAPD, and may be subject to change. Learn what is included in the Rialytix data. Please refer to the firm's official disclosures for the most accurate information using their CRD number. This is not investment advice.