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Day & Zimmermann
Harold L. Yoh III runs Day & Zimmermann, the 125-year-old family-held industrial group behind major US munitions and power-plant maintenance contracts.
Day & Zimmermann
Day & Zimmermann was founded in 1901 by engineer Charles Day and has stayed under family control for four generations. The Yoh family built the firm from a single engineering practice into a diversified industrial services group; Harold L. Yoh III now leads the enterprise from its Philadelphia headquarters. The firm operates across four segments — Maintenance & Construction, Munitions, Government Services, and Workforce Solutions — that collectively cover nuclear plant modifications, fossil and renewable power projects, direct-fire munitions manufacturing, embassy security, and national intelligence support. Its Maintenance & Construction division serves power, chemical, and industrial plant owners, while the Munitions unit is one of the largest private ammunition producers for the US Department of Defense. A dedicated architecture and engineering subsidiary, Mason & Hanger, has designed a 426-acre federal law enforcement training campus and secured a renewed US diplomatic presence in a high-threat region of Africa. The Workforce Solutions arm, operating via the Yoh brand, places technical talent across critical-infrastructure and IT programs. May 2026: Day & Zimmermann was ranked among Engineering News-Record's Top 500 Design Firms, continuing a multiyear pattern of recognition in the federal and industrial engineering peer group. The firm lists additional offices on its website though specific locations are not enumerated. The group's giving vehicle, D&Z Gives Back, funnels philanthropic activity through the operating company rather than a structurally separate foundation. Unlike a typical family office that allocates liquidity into third-party funds, Day & Zimmermann retains and reinvests earnings directly inside a holding company that itself executes security-cleared, long-cycle government and industrial contracts. That architecture makes the Yoh family's wealth inseparable from the operating business — there is no separate investment portfolio, no external LP base, and no co-investment club.
General information
Firm type
Single Family Office
Year founded
1901
AUM
Undisclosed
Location
Region
North America
Country
United States
City
Philadelphia
Corporate office
Philadelphia, PA, United States
Principals
Harold L. Yoh, III
Chairman and CEO
Sector focus
Frequently asked questions
Who controls Day & Zimmermann and how is the family involved?
The Yoh family has controlled Day & Zimmermann since H.L. Yoh joined the firm in 1936. Harold L. Yoh III currently serves as Chairman and CEO. Several other family members hold leadership roles, including Mike Yoh and Daniel Ross. The firm is treated as a family-held industrial enterprise — there is no outside LP base or separated investment portfolio.
How does Day & Zimmermann generate returns for the family?
Rather than earning management fees on outside capital, Day & Zimmermann generates returns through the operating profits of its four divisions: Maintenance & Construction, Munitions, Government Services, and Workforce Solutions. It is an operating company, not an investment manager. Family wealth is tied directly to the firm's success as a government and industrial contractor.
Does Day & Zimmermann operate as a typical family office that allocates to external funds?
No. Day & Zimmermann does not disclose a liquid investment portfolio, fund-of-funds activity, or external capital-allocation function. Its structure more closely resembles a privately held industrial conglomerate than a single-family office — profits are retained and reinvested inside the operating company rather than deployed into third-party assets.
Which US government agencies are Day & Zimmermann's main clients?
The firm's Munitions division manufactures, stores, and demilitarizes ammunition products primarily for the US Department of Defense and approved foreign governments. The Government Services segment provides integrated security, architecture and engineering, and intelligence support to US embassies, law enforcement, and the Navy and Marine Corps.
How is the philanthropic activity structured?
Day & Zimmermann channels its charitable giving through the D&Z Gives Back program, which is part of the operating company. The firm does not appear to maintain a structurally separate family foundation, keeping its community investments integrated with the corporate entity.
What is the Mason & Hanger relationship to Day & Zimmermann?
Mason & Hanger is listed as 'A Day & Zimmermann Company' and functions as the firm's architecture and engineering subsidiary for US government and commercial clients. It has designed a 426-acre law enforcement training campus and provided A&E services for secure diplomatic facilities abroad.
Does Day & Zimmermann have a venture capital or growth equity arm?
There is no disclosed venture capital, growth equity, or independent financial-investment vehicle. The closest parallel is the Yoh.com Workforce Solutions segment, which places technical talent, but this is an operating staffing business, not a principal-investing function.
Profile maintained by Altss using OSINT (open-source intelligence), regulatory filings, licensed data partners, and verified direct submissions. Read the methodology. Last updated: . Continuous refresh with full update cycles at least every 30 days.
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