Ending a six-month search, the Prince William Board of County Supervisors has picked James H. Mullen, deputy city manager of Aurora, Colo., to take over the county's top administrative post and is expected to announce his appointment tomorrow, sources said during the weekend.

Prince William, a rapidly growing county of 230,000, has been without a permanent county executive since December, when Robert S. Noe Jr. resigned to join the Anden Group, a developer with substantial real estate holdings in the county.

Noe's successor will take the job as the county, the second-largest jurisdiction in Virginia, struggles to provide services to its increasing number of residents and is on the verge of modifying its political structure.

Mullen has been deputy city manager for Aurora, a city of 230,000 people outside Denver, since 1986. He supervised the city's public services department, including police programs and aid for the indigent. Aurora's government includes 1,900 employees and a $90 million budget.

Mullen said in a telephone interview yesterday that he was attracted to Prince William because "it's a fast-growing county with a reputation for being well-managed." The board has asked him to begin work in mid-August, he said.

The supervisors voted in closed session to offer Mullen the county executive post nearly two weeks ago and held a special closed meeting Wednesday to approve a salary and benefits package offer, according to the sources.

A former U.S. Air Force captain, Mullen, 47, and his wife, Pat, said they would fly to Washington yesterday. Mullen said he would not formally accept the job until he had met with the supervisors one more time and he and his wife had checked out area schools for the younger of their two sons, who is 7 years old.

Sources said Mullen would receive a salary of close to $100,000 a year as county executive. Noe, who held the post for 11 years, made $92,800 in 1989, and the salary of the highest-paid county employee, Schools Superintendent Edward S. Kelly, is $97,000. Mullen's salary in Aurora is $72,000.

The new county executive will join Prince William at a difficult time. The cooling Washington-area real estate market means that the county can no longer count on rising property values to increase its $295 million budget and pay for new programs. At the same time, population growth -- more than 55 percent in the 1980s -- has left schools, roads and facilities burdened.

The new executive also faces the possibility that the composition and dynamics of the board to which he reports will change radically after next year's elections. Prince William voters are scheduled to elect a county-wide chairman for the first time, and the supervisors will probably create another magisterial district, bringing the board's membership to nine.

The nationwide search drew more than 90 applicants, and an Atlanta-based search firm brought 10 candidates to the supervisors for interviews.

The supervisors narrowed the choice to seven finalists last month. The other finalists were Prince William's acting county executive, Connie Bawcum; Gary Gwyn, city manager of Tyler, Tex.; Rodney L. Kendig, city manager of Pensacola, Fla.; Elmer C. Hodge Jr., county administrator of Roanoke; Marie Amie Shook, assistant county manager of Mecklenburg County, N.C.; and David Richard Mora, city manager of Oxnard, Calif.

Bawcum drew strong support from two supervisors -- Edwin C. King and Kathleen K. Seefeldt -- but several other board members opposed her because they considered her too closely tied to Noe. Bawcum is expected to continue working in Prince William as deputy county executive.

Previously, Mullen was city administrator of Greenwood Village, Colo., and worked in the city governments of Casper, Wyo., and Charleston, S.C. He holds a master's degree in public administration from the University of Colorado and a bachelor of science degree from the U.S. Air Force Academy.