The cuts, mostly of back-office workers, amounts to more than 10% of Pershing Square Capital Management's staff, according to the Journal.
The layoffs are unrelated to the hedge fund's recent poor performance, but rather the firm's ability to automate back-office tasks like filling out investor forms, according to the report.
A Pershing Square spokesman declined to comment.
Pershing Square has faced a decline in performance this year, as a big investment in Valeant Pharmaceuticals soured. Ackman has said that he expects the investment to rebound.
Pershing Square's publicly traded vehicle, which mimics the private hedge fund, is down 20.9% this year through June 21. The New York activist firm had its worst year ever last year, slumping 20%.
With performance declines, assets under management have also dropped. The firm managed $20.2 billion at its peak last July, and about $12 billion as of May.
Pershing Square's structure has helped the firm retain assets by preventing a run on its reserves during rough times. Outside investors can only pull one-eighth of their capital every quarter, for example. And the firm's $4 billion closed-end vehicle is another source of permanent capital, since investors have to sell their shares to redeem.