The Thompson Block is located in Portland's Old Port area, on the north side of Middle Street and at the head of the now-demolished Deer Street.[2] It is flanked on the left side by the Rackleff Building, which stands across Church Street, and by a less ornate brick commercial building on the right, across a narrow alley. It is a 3+1⁄2-story brick building, roughly trapezoidal in shape, with a polychrome slate mansard roof providing a full fourth floor. The ground floor consists of an arcade of iron supports, forming arches over either display windows or recessed store entries. A stone entablature separates the ground floor from the next two floors. These floors are divided into six bays, articulated in a 1-1-2-1-1 pattern by stone piers, and each floor is also delineated horizontally by rusticated stone beltcourses above and below the windows. Windows on the second floor are paired round-arch windows with keystones, while third-floor windows have segmented-arch tops with stone hoods. Two of these are more ornate, rising into wall dormers that interrupt the bracketed and dentillated cornice. Dormers at the roof level have round-arch windows with keystoned and eared hoods.[3]
The block was built in 1867–1868, in the wake of Portland's great 1866 fire. George M. Harding, who designed the Thompson Block and the adjacent Rackleff and Woodman buildings, played a major role in the rebuilding effort, and this trio of buildings represent some of his finest commercial work. Urban renewal of the 20th century resulted in the loss of a significant number of other period buildings, and this is one of the few places in the city where the typical rebuilt post-fire streetscape is to be seen.[3]