After studying in the private law office of two established attorneys, Bleecker attained admission to the bar in 1801, and began a practice in Albany.

He became a highly regarded lawyer and teacher, and many men who went on to prominent legal and business careers learned the law under his tutelage.

Bleecker took part in numerous philanthropic and civic endeavors, including the establishment of several Albany-area hospitals and oversight of several universities.

[3] As a young man, Harmanus Bleecker studied law with John Vernon Henry and James Emott,[4] was admitted to the bar in 1801, and commenced practice in Albany.

[7] Bleecker was also a highly regarded teacher, and among the students who learned the law in his office were: David Dudley Field (1805–1894);[8] Stephen Johnson Field;[9] Charlemagne Tower;[10] Bradford R. Wood;[11] Peter Gansevoort;[12] Solomon Southwick;[13] Timothy Childs,[14] and Charles Fenno Hoffman.

Bleecker also pursued several business ventures, including serving on the board of directors of Albany's Mechanics and Farmers Bank.

[26][27] He was Chargé d'Affaires to the Netherlands from May 12, 1837 to June 28, 1842, initially appointed by President Martin Van Buren, who was friendly with Bleecker and shared his interest in Dutch culture and language.

[30] According to often-repeated accounts, when Bleecker presented his credentials, King William supposedly remarked "You speak better Dutch than we do in Holland!

[35][36] Harmanus Bleecker maintained numerous friendships and professional relationships, many of which transcended political leanings.

[39] In addition, Bleecker was a longtime friend of Martin Van Buren, a Democrat with whom he shared an interest in the Dutch culture and language.

[43] Bleecker married Sebastiana Cornelia Mentz of Arnhem, a woman many years younger than him, whom he met while he was a diplomat in the Netherlands.

[47][48][49] Originally located near the corner of Washington Avenue and Lark Street, the 1889 Bleecker Hall was used until 1919, when the Young Men's Association, which was responsible for the site, decided to create a structure for use as a library only.

[51] In more recent times the building has been renovated as private office space, and it is listed on the National Register of Historic Places.