Mercer is located between 44°36'30” and 44°43' north latitude and is between 69°51' and 69°58' west longitude. It lies at the extreme southwestern corner of Somerset County where 3 towns and 3 counties all come together atop Hampshire Hill. Somerset, Franklin and Kennebec Counties along with Mercer, Rome, New Sharon all share this point. Mercer has 5 sister towns for neighbors: Norridgewock and Smithfield on the East, Rome to the South, New Sharon and Starks on the West and Starks and Norridgewock on the North.
SizeMercer has an area of about 28.13 square miles or 18,003 acres. When compared to its 5 sister towns, it ranks fourth in size. Mercer's greatest width is about 5 1/2 miles; and its greatest length is about 7 1/2 miles. The northern border is very irregular because of the meandering Sandy River which forms a natural boundary. The boundary between Mercer and Norridgewock is rather angulated because of two farms which were set off from Mercer to Norridgewock.