Milliken studied at the Shawnigan Lake School. After finishing high school, he went to the University of Alberta to pursue a B.A. in Economics and Philosophy, and a B.Com., Strategic and Organization with distinction. He then received a law degree from the University of Saskatchewan.^[6] While he was studying for his first degree, he worked as an assistant account manager at GE Capital. After graduating from law school, he started his career as a lawyer at Dinning Hunter Lambert & Jackson, a law firm in Victoria, BC, where he worked for a year and five months. He then moved to another full-service law firm, Davis LLP (DLA Piper), to continue his profession. After working for a year at that firm, he became a legal recruitment consultant at a legal recruitment company in Calgary. A year later, he worked for Brydges Line Duty Counsel as a legal counsel for 5 years and 4 months. After that, he decided to quit that job and start his own legal recruitment company, Brolly Legal Recruitment in Calgary. He expended the company to Toronto while serving as the founder and CEO from April 2013 to April 2019.^[7]

The Minister of Mental Health and Addictions, Milliken, and the Government of Alberta have pledged $1.8 million towards pre-treatment programs aimed at providing addiction support in Calgary. The funding will go towards a pre-treatment program designed to bridge the gap between detox and treatment and will assist up to 240 Albertans every year in their journey towards recovery.^[8]

The Minister of Mental Health and Addictions, Milliken, has played a pivotal role in the government of Alberta's plan to make large investments in mental health and addiction treatment in the forthcoming Alberta budget 2023. The aim is to provide support to a larger number of Albertans in their pursuit of recovery. The budget will allocate more than $275 million towards addiction and mental health, which is the highest amount ever recorded in the region.^[9]

This significant increase in funding is in contrast to the mental health and addiction-specific budget of $87 million per year, recorded in 2019 when the current government assumed office. The Minister's commitment to improving the quality of life for all Albertans, regardless of age, is evidenced by the record investments in addiction and mental health. If passed, Budget 2023 will provide $275 million in funding for mental health and addiction treatment.^[10]

In July 2020 Milliken came under fire for, while sitting as Speaker, ejecting NDP member Marie Renaud out of the chamber during debate. Renaud noted that UCP members were bullying her while she was standing to speak to a bill. When Renaud refused to apologize for her comments, she was ejected. Milliken's impartiality was called into question since he focused the conflict on Renaud while failing to address the UCP members' behaviour.^[11]

On Sunday January 8, 2023 Milliken posted a tweet from a British tabloid paper that was widely considered to be problematic and reflected a "bias against these individuals and a complete misunderstanding around the nature of addiction and homelessness". He took it down but when challenged on it reposted it with the headline highlighted before taking it down a second time. In a CTV article Lorian Hardcastle, a University of Calgary associate law professor specializing in health policy, characterized the tweet by the minister's account as "highly problematic," suggesting a potential misunderstanding of addiction and homelessness.^[12]