He was the planning commission's director from 1943 to 1948 and made the preliminary plans for an expressway system, the building of Penn Center, the park around Independence Hall, and the development of Society Hill. He brought capitol programming and budgeting to Philadelphia for the first time.
Martin Meyerson, former president of the University of Pennsylvania and a city planning colleague of Mr. Mitchell's, said, "At the end of World War II, Philadelphia was the last major American city to establish an overall planning agency. Robert Mitchell was its first director. In one year he created the most distinguished post of that kind in the United States."
After leaving his office as director, he went to Columbia University as a research professor in urban land use and housing. He came back to Philadelphia in 1951 to initiate and chair the department of city planning at Penn.
He became professor emeritus after retiring in 1975. He joined the university's newly formed Dynamics of Organization program in 1984 and taught three semesters a year in Decision Making Systems.
Mr. Mitchell was the director of the Urban Traffic and Transportation Board under Mayor Joseph Clark during the mid-1950s.
He was either a consultant, founder, or participant in many city planning, urban renewal and transportation boards.
He was a participant in founding and chairman of the Planning and Development Collaborative International in Washington from 1965 to 1975.
He was also the primary consultant to the New York City Planning Commission for 10 years.
Mr. Mitchell was consultant at various times to governments and development agencies in Philadelphia; the Chicago area; California; Oahu, Hawaii; Montreal; Singapore; Ciudad Guyana, Venezuela; Sao Paulo, Brazil; Mexico City; Madrid; Amman, Jordan; Egypt; Tanzania, and Botswana.
He was also one of the founders of The West Mount Airy Neighbors, a community organization, and belonged to many other organizations, including the American Planning Association, the American Institute of Certified Planners, the Institute of Noetic Sciences, and the Faculty Club of the University of Pennsylvania.
He was a co-author of Urban Traffic, a Function of Land Use in 1954, and editor of several volumes of Annals of the American Academy of Political and Social Science.
Born in Indianapolis, Ind., Mr. Mitchell received a bachelor's degree in architecture from the University of Illinois in 1928. He also did graduate work in urban studies at the university. He received an honorary master's in architecture from the University of Pennsylvania in 1975.
He was an instructor, research professor, professor or director in urban or architectural programs at the University of Illinois, Columbia University and the University of Pennsylvania.
Lois Ginsberg, associate director of Penn's Dynamics program, said that Mr. Mitchell "was an exceptional teacher, public servant, philosopher, humorist, and international planner."
An avid reader and thinker, Mr. Mitchell loved playing popular music of the 1920s and '30s on the piano. Recently, while being honored at a dinner at Penn, he told the audience that if he had a piano he would play two songs: ''I'll Be Loving You Always" and "One of These Days, You're Going to Miss Me, Honey."
He was the father of John Robert Mitchell, who died 12 years ago.
He is survived by his wife of 59 years, Bernice Anderson Mitchell; a daughter, Janet Mitchell Krejs, and a granddaughter, Christiane M. Krejs.
A memorial service will be held at 2 p.m. Dec. 18 at the Unitarian Church of Germantown, 6511 Lincoln Drive.
In lieu of flowers, memorials may be made to the charity of choice.