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Merion Realty Advisers
MainLine Investment Partners was founded in 2010 by William Landman and David Clapper as a family office vehicle. Landman brought a transactional legal...
Merion Realty Advisers
MainLine Investment Partners was founded in 2010 by William Landman and David Clapper as a family office vehicle. Landman brought a transactional legal background from his prior partnership at Reich & Landman, while Clapper arrived from Ernst & Young’s assurance and advisory practice, where he covered real estate, private equity, and hedge fund clients. The firm’s real estate platform, Merion Realty Partners, was established in 2011 as a direct spinout of the Real Estate Division of CMS Companies, giving it an immediate institutional operating history separate from a typical start-up manager. The firm splits its capital between direct private equity in operating businesses and multifamily real estate. The real estate strategy is channeled through Merion Realty Partners, which describes itself as a vertically integrated investment and property management platform. On the private equity side, MainLine takes control-oriented positions in operating companies. One confirmed public holding is Zeta Global Holdings Corp, a marketing technology company listed on the NYSE, where William Landman serves on the board. The firm states that its investments are typically financed with its own equity alongside capital from its network of outside institutional and high-net-worth investors. Geographically, the real estate platform operates under the Merion Residential brand, focusing on domestic US multifamily assets. Team size and total AUM are not publicly disclosed. The headquarters remain in Wynnewood, Pennsylvania, with no additional offices listed. Beyond the founding partners, the leadership team includes Chief Compliance Officer Donna Rittershausen, a 25-year regulatory veteran who previously served in fund administration and CCO roles at CMS. In June 2024, Rittershausen transitioned from MainLine to a Compliance Director role at 3iCO, LLC, though she retains her CCO title at MainLine Investment Advisers. Landman’s advisory relationship with CW Advisors continued until its 2025 acquisition by Osaic, Inc. The structural differentiator is the dual-platform architecture under a single family office umbrella. Rather than allocating to external funds, MainLine operates its own real estate investment and property management company — Merion — while running a separate direct private equity book. This design blurs the line between a family office and an owner-operator, giving the principals hands-on control of both deal selection and asset management without the intermediation of a third-party GP.
General information
Firm type
Single Family Office
Year founded
2010
AUM
Undisclosed
Location
Region
North America
Country
United States
City
Wynnewood
Corporate office
308 E. Lancaster Avenue, Suite 300, Wynnewood, PA 19096, United States
Principals
William Landman
Co-Founder and Managing Partner
David Clapper
Co-Founder and Partner
Donna Rittershausen
Chief Compliance Officer
Sector focus
Frequently asked questions
Who runs investment decisions at MainLine Investment Partners?
Co-founders William Landman and David Clapper are the controlling members and direct the investment and management strategies. Mr. Clapper focuses the majority of his time on deal origination and investment selection, while also overseeing the operating businesses where substantial investments have been made. Mr. Landman serves on portfolio company boards, including publicly traded Zeta Global.
What is the relationship between MainLine Investment Partners and Merion Realty Partners?
Merion Realty Partners was established in 2011 as a real estate investment and management platform spun out from the Real Estate Division of CMS Companies. It operates as a vertically integrated affiliate of MainLine Investment Partners. The two co-founders are controlling members of both entities, and Merion runs multifamily assets under the Merion Residential brand.
Does MainLine invest in funds or make direct investments?
MainLine makes direct investments, not fund commitments. On the real estate side, Merion Realty Partners pursues and operates multifamily properties directly. On the private equity side, the firm takes control-oriented stakes in operating businesses, financing deals with its own equity capital plus contributions from outside institutional and high-net-worth investors in its network.
What does MainLine's real estate platform actually manage?
Merion Realty Partners is a vertically integrated platform that handles both investment and day-to-day property management for its multifamily residential portfolio. The operating brand, Merion Residential, focuses on providing a simple rental experience, suggesting a strategy centered on stabilized, income-producing apartment properties rather than ground-up development.
Can external investors access MainLine's deals?
The firm states that its investments are typically financed with equity from MainLine itself plus capital from 'certain outside institutional and high net-worth investors from our network.' This suggests a co-investment club model where access is relationship-driven rather than through open-ended commingled funds.
What is William Landman's background outside of MainLine?
William Landman was previously a partner at Reich & Landman, a law firm with offices in Pittsburgh and Sarasota, where he focused on transactional corporate work and representing professional athletes. He is an Emeritus Trustee of the Thomas Jefferson University Board of Trustees and served as a senior advisor to CW Advisors until its acquisition by Osaic in 2025.
How is the firm's compliance infrastructure structured?
The firm employs a dedicated Chief Compliance Officer, Donna Rittershausen, who has specialized in SEC and FINRA regulatory compliance for more than 25 years. She holds FINRA securities licenses and has served as CCO at MainLine Investment Advisers since 2021. In June 2024 she also took a Compliance Director role at 3iCO, LLC, an affiliated entity.
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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