Prior to entering state parliament, McLachlan was a lawyer, army officer and a businessman working in the financial services industry.

He also served on the national executive of the Trustee Corporations Association of Australia and on the compliance and investment boards of the Financial Services Council.

In late 2015 McLachlan introduced a Bill that would give legal protections to the identity of individuals who provide sensitive information to journalists.

This is a common protection in other Australian jurisdictions, however; in South Australia the current law requires journalists to identify sources during court proceedings or commissions of inquiry, otherwise face a potential prison sentence.

[13][14] At the time of moving his successful amendment, McLachlan told Members of the Legislative Council: In the land of questionable regimes that are often founded on intolerance, Israel has a vibrant democracy that operates under rule of law and is innovative and prosperous.

McLachlan defeated former Law Council of Australia president Morry Bailes and former state party treasurer Michael van Dissel.

[15][16] He resigned as president and a member of the Legislative Council on 5 February 2020, and was appointed to the Senate by a joint sitting of the South Australian Parliament the following day.