William Blair Files 13G/A Showing 4.7M Mercury Systems Shares

Rhea-AI Filing Summary

William Blair Investment Management, LLC filed Amendment No. 8 to Schedule 13G disclosing its beneficial ownership of Mercury Systems, Inc. (MRCY) common stock as of 30 June 2025.

• Owns 4,695,683 shares, equal to 7.9 % of the outstanding class.
• Holds sole voting power over 4,375,808 shares and sole dispositive power over the full 4,695,683 shares; no shared authority reported.
• Filed under Rule 13d-1(b) as a registered investment adviser (IA) organized in Delaware.
• Certification states the position is held in the ordinary course of business and not to influence control of the issuer.

The filing confirms a sizable institutional stake but does not provide financial performance data or signal strategic changes at Mercury Systems.

Insights

Financial Analyst

TL;DR: William Blair reports 7.9 % ownership in MRCY; informative but operationally neutral.

William Blair Investment Management’s 13G/A shows continued institutional involvement with 4.7 million shares. While the stake surpasses the 5 % disclosure threshold—highlighting Mercury Systems’ appeal to asset managers—the filing carries no intent to influence control, nor does it outline purchase timing or cost basis. Consequently, the disclosure is informational rather than catalytic; it neither alters Mercury’s fundamentals nor provides guidance on future trading. Investors may view stable institutional support as mildly constructive, yet the absence of transactional context limits immediate valuation impact.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 8)

MERCURY SYSTEMS INC

(Name of Issuer)

Common Stock

(Title of Class of Securities)

589378108

(CUSIP Number)

06/30/2025

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

Rule 13d-1(b)

Rule 13d-1(c)

Rule 13d-1(d)

SCHEDULE 13G

CUSIP No. 589378108

1

Names of Reporting Persons

WILLIAM BLAIR INVESTMENT MANAGEMENT, LLC

2

Check the appropriate box if a member of a Group (see instructions)

(a)
(b)

3

Sec Use Only

4

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 4,375,808.00
6	Shared Voting Power 0.00
7	Sole Dispositive Power 4,695,683.00
8	Shared Dispositive Power 0.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

4,695,683.00

10

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

11

Percent of class represented by amount in row (9)

7.9 %

12

Type of Reporting Person (See Instructions)

IA

SCHEDULE 13G

Item 1.
(a)	Name of issuer: MERCURY SYSTEMS INC
(b)	Address of issuer's principal executive offices: 50 MINUTEMAN ROAD, 50 MINUTEMAN ROAD, ANDOVER, MASSACHUSETTS, 01810.
Item 2.
(a)	Name of person filing: William Blair Investment Management, LLC
(b)	Address or principal business office or, if none, residence: 150 North Riverside Plaza, Chicago, IL 60606
(c)	Citizenship: Delaware
(d)	Title of class of securities: Common Stock
(e)	CUSIP No.: 589378108
Item 3.	If this statement is filed pursuant to §§ 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:
(a)	Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o);
(b)	Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);
(c)	Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c);
(d)	Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8);
(e)	An investment adviser in accordance with § 240.13d-1(b)(1)(ii)(E);
(f)	An employee benefit plan or endowment fund in accordance with § 240.13d-1(b)(1)(ii)(F);
(g)	A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);
(h)	A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);
(i)	A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);
(j)	A non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J). If filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J), please specify the type of institution:
(k)	Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K).
Item 4.	Ownership
(a)	Amount beneficially owned: 4,695,683
(b)	Percent of class: 7.9%
(c)	Number of shares as to which the person has:
	(i) Sole power to vote or to direct the vote: 4,375,808
	(ii) Shared power to vote or to direct the vote: 0
	(iii) Sole power to dispose or to direct the disposition of: 4,695,683
	(iv) Shared power to dispose or to direct the disposition of: 0
Item 5.	Ownership of 5 Percent or Less of a Class.
Item 6.	Ownership of more than 5 Percent on Behalf of Another Person.
	Not Applicable
Item 7.	Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person.
	Not Applicable
Item 8.	Identification and Classification of Members of the Group.
	Not Applicable
Item 9.	Notice of Dissolution of Group.
	Not Applicable

Item 10.

Certifications:

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under § 240.14a-11.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

WILLIAM BLAIR INVESTMENT MANAGEMENT, LLC

Signature:	Walter R. Randall, Jr.
Name/Title:	Chief Compliance Officer
Date:	07/30/2025

FAQ

How many Mercury Systems (MRCY) shares does William Blair own?

4,695,683 shares are beneficially owned by William Blair Investment Management.

What percentage of Mercury Systems stock is held by William Blair?

The firm reports ownership of 7.9 % of the outstanding common stock.

Does William Blair have sole voting power over its MRCY shares?

Yes, it has sole voting authority over 4,375,808 shares and no shared voting power.

Under which SEC rule was this Schedule 13G/A filed?

The statement was filed pursuant to Rule 13d-1(b) as an investment adviser.

What is the event date for William Blair’s ownership report?

The relevant event date is June 30, 2025.

Is William Blair attempting to influence control of Mercury Systems?

No. The certification states the shares are held in the ordinary course of business and not to influence control.

What is the CUSIP for Mercury Systems common stock?

The CUSIP number is 589378108.