In 1854 and again from 1859 to 1861, Jackman represented Ward 6 on the Newburyport Board of Alderman. On February 27, 1861, Jackman was chosen to succeed the deceased Moses Davenport as Mayor. On April 19, 1861, Jackman and a committee of citizens arranged Newburyport's first citywide memorial service following the assassination of Abraham Lincoln. Jackman was elected to his own term in 1862 and served again from 1864 to 1865. From 1871 to 1872 he served another stint on the Board of Aldermen. In 1877, Jackman was elected to his fourth and final term as mayor.