The town is officially transitioning into new leadership. Robert E. Mercier was set to retire as town administrator on Friday, and his successor, John D. Petrin, is due to begin work Monday. Mercier had been town administrator since 1999 after having previously held the post from 1980 to 1986. Petrin, Ashland’s town manager since 2005, was chosen by selectmen in April from among three finalists for the Burlington job, which attracted 22 applicants. Prior to assuming his Ashland job, Petrin, a Bolton resident, served as assistant school superintendent in Marlborough from 2001 to 2005; town administrator in Harvard from 1988 to 2001; town administrator in Pepperell from 1985 to 1988; and town assessor in Bellingham from 1982 to 1985.