He was the second son of Elizabeth (née Bland) and Henry Haswell of Presteigne in Radnorshire, Wales.

[1] On 20 July 1841, Halswell was appointed chairman of the courts of quarter sessions, "Protector of Aborigines in the Southern District of this Island" (i.e. Māori people), and Commissioner for the Management of the Native Reserves.

[4] On 26 February 1842, he was appointed judge of the County Court of the southern portion of New Ulster Province.

[3][9] On 3 May 1841, Halswell was appointed to the General Legislative Council as one of the country's three senior justices.

[13] On 24 April 1845, Halswell attended a farewell function for James Sea, who was to go to New South Wales to become the first manager of the Sydney branch of the Union Bank of Australia.

[18] Halswell was one of the founding members of the Canterbury Association; he attended the inaugural meeting on 27 March 1848 and was on the initial committee.

[3] Halswell's eldest son, (Edward) Darnley Halswell, was killed aged 19 in January 1842 in Sindh (now part of Pakistan) whilst part of General Charles James Napier's campaign to conquer the area.

[22] A rural settlement near the head of this river then adopted the name Halswell; this is today a suburb of Christchurch.

When the land was subdivided, the street was initially named for its original owners but later renamed.