After the war he became a founding partner of the predecessor firm to DeLeuw Cather and Associates (Kelker, DeLeuw and Co.).  From 1933 to 1935 he was Assistant Chief Highway Engineer for the Illinois Department of Public Works and Buildings and from 1935 to 1936 Director of the Illinois Statewide Highway Planning Survey.  Starting in 1938 Mr. DeLeuw became Chief Subway Engineer for the City of Chicago.  During the war years he worked with problems involving Chicago's Transit System and the war effort and also served as Chief Engineer for the City's Superhighway Commission.  Much of the planning for the Chicago Expressway System evolved as a result of his effort.

The firm of DeLeuw Cather and Associates, one of the nation's largest during Mr. DeLeuw's tenure, grew into an international consulting firm of 1,500 members.  Mr. DeLeuw was registered as a professional engineer in 36 states.