He began his studies at Uppsala University in 1722, after which he served as an extra clerk on a judicial inspection trip in 1728.

After moving to the Gotland artillery battalion, Mannerheim was promoted to captain in 1741, to major in 1747 and eventually to lieutenant colonel in 1750.

[3][4] Mannerheim was initially announced as married to his fiancée, the chamberlain's daughter Margareta Elisabet Nordenflycht (1714–1748), but this ended in divorce in 1744.

Finally, in 1747, he married the lieutenant colonel's daughter Helene Maria Söderhjelm (1722–1793) in the Västervåla Church [sv].

They had children Anna Maria (1748–1839), Lars Augustin [sv] (1749–1835), Gustava Charlotta (b.