A partner in the national firm Simpson Grierson from 1983, Sir Mark was admitted to the bar in 1979. He practised in Auckland, serving as principal legal advisor for Auckland City Council, North Shore City Council, and Rodney District Council. In 1997 he commenced practice as a barrister sole from chambers in Auckland. In 2000 Sir Mark was appointed Queen’s Counsel, among the first Māori to take silk. 

In March 2004, Sir Mark was appointed to the High Court, sitting in Auckland. Following the Canterbury earthquakes (2010–2011), he was named chair of the Royal Commission of Inquiry into Building Failures caused by the earthquakes. Between May 2011 and November 2012, he chaired 33 public hearings and led the Commission to deliver four reports under intense time pressure and public scrutiny, navigating high volumes of evidence including technical, structural, and earthquake engineering evidence.

Sir Mark became a Judge of the Court of Appeal in September 2014 and was appointed President of the Court in 2022. 

In October 2024, Sir Mark was granted the right to retain the title “The Honourable” for life in recognition of his service as a Judge. Recognized as a trailblazer for Māori lawyers with strong expertise in resource management and local government law, Sir Mark retired from the judiciary in November 2024 upon reaching the mandatory retirement age of 70. In the 2025 King’s Birthday Honours, Sir Mark was appointed a Knight Companion of the New Zealand Order of Merit for services to the judiciary. 

Sir Mark chaired the Council of Legal Education from 2018 to 2024 and was founding chair of the judiciary’s open justice committee, Huakina kia Tika. He was heralded “the architect and catalyst” for the introduction of tikanga education into Aotearoa New Zealand’s law schools. 

“It’s a pleasure to be back in Auckland and reconnecting with the profession here after an extended period in Wellington,” says Sir Mark.

Sir Mark’s whakapapa includes the Waikato-Tainui iwi Ngāti Māhanga and English, Scottish, and Irish ancestry.