Kerry Watson, a former Prince George’s police officer who has worked as a top adviser to Prince George’s County Executive Rushern L. Baker III (D) has left Upper Marlboro to join Alexander & Cleaver, a law firm that does extensive lobbying in Annapolis.

Watson, who served 20 years on the county police force, said he hopes to boost the firm’s presence in Prince George’s and also will do general lobbying for the firm’s clients.

Watson recently worked as Baker’s liaison to the Prince George’s County Council, where he helped Baker push through legislation on ethics, gaming and economic development.

One item of unfinished business Watson has left for his colleagues: the nomination of Darrell B. Mobley to head the county’s public works and transportation agency. Council chairman Andrea C. Harrison (D-Springdale) stalled that appointment during the summer, saying she wanted Mobley to spend more time as acting director because she was concerned he did not know all aspects of the job. Mobley is a former acting secretary of transportation in the O’Malley administration.

In a statement issued by Alexander & Cleaver, Baker said “Kerry provided very keen insight and dedicated service as a key member of my staff, and he will be missed.” Lisa Jackson, who heads Baker’s intergovernmental affairs office, will split duties with other staffers to cover the County Council until a replacement is hired.

Watson is the second new hire announced this week by Alexander & Cleaver. Former State Sen. Rob Garagiola (D-Montgomery), the former Senate majority leader, has also joined the firm, and says he will help focus on Montgomery County.