He was active in organizing the Ancient and Honorable Artillery Company, of which he became captain in 1640. In 1652 he was appointed commander of all the Massachusetts militia. He ordered the building of Castle William, the first fort at Boston, and Oliver Cromwell promoted him to major general, making him the first Major General of the Massachusetts Bay Colony. With John Winthrop, Jr. and others, he established in 1643-1644 the first ironworks in North America.
Under authority from Cromwell, in July 1654 Sedgwick raided Acadia by conquering the French at Fort Pentagouet, Maine; Fort La Tour at the mouth of the St. John River; and Port Royal. In 1655 he accompanied the expedition which captured Jamaica. Just before his death there, Cromwell gave him sole command as the first Governor General of Jamaica. He died on 24 May 1656, and was buried halfway up the canyon to Spanish Town from Ocho Rios in a small Christopher Wren-styled chapel.
He was the great-great-grandfather of Theodore Sedgwick (1746–1813), the twice eighth great-grandfather of Edie Sedgwick and the twice ninth great-grandfather of Kyra Sedgwick and James Hamilton Geer, Jr., Esq.. Sedgwick, Maine, was named in his honor.
- Records of Saint Mary's Church, Woburn, Bedfordshire, England
- SEDGWICK.ORG - Major General Robert Sedgwick (1613 – 1656)
- This article incorporates text from a publication now in the public domain: Gilman, D. C.; Thurston, H. T.; Moore, F., eds. (1905). "article name needed". New International Encyclopedia (1st ed.). New York: Dodd, Mead.