Among his works were an early stereotype edition of Scott's Family Bible, which was very popular, and The Panoplist, a religious magazine devoted to missionary interests.

He was active in Boston politics during the 1820s, twice winning a seat in the Massachusetts General Court (state legislature).

[3] Another major success was his printing of the first stereotype edition of Scott's Family Bible, a highly popular work that sold tens of thousands of copies.

In 1818, upon the end of their apprenticeship, Armstrong turned over operation of the printing business to them (which then became known as Crocker & Brewster) and focused his activities on the bookshop.

[15] He was, however, elected lieutenant governor on the Whig ticket, serving first under Levi Lincoln Jr. and then John Davis.

[16] When Davis resigned in March 1835 upon his election to the United States Senate, Armstrong served as the Acting Governor until 1836.

[9] In the 1836 campaign Armstrong sought the Whig nomination for governor, but it went instead to Edward Everett in a bid for support from the Anti-Masons.

Armstrong ended up running that year without party support,[17] and came in a distant third behind Everett and perennial Democratic candidate Marcus Morton.

[9] The principal act of civic improvement during his one-year administration was the construction of iron fencing around the Boston Common and the widening of the promenade along Boylston Street.