Winkelmann began work as a law clerk with Auckland firm Nicholson Gribbin. In 1988, at age 25, Winkelmann became the first female partner and one of the youngest partners ever in the firm's then 117-year history. She remained at that firm until May 2001 when she began practice as a barrister sole specialising in insolvency, commercial litigation and medical disciplinary litigation. Winkelmann was appointed a High Court Judge in July 2004 by which time the youngest of her four children, twins, were aged five. She was appointed as Chief High Court Judge with effect from 1 February 2010. She remained in that position until her appointment to the Court of Appeal in 2015. As Chief High Court Judge, Winkelmann introduced reforms aimed at improving accessibility to the High Court's processes in its civil jurisdiction, improving the timeliness with which the Court dealt with both civil and criminal matters, and improving public understanding of the work of the Courts. These initiatives included the reintroduction of the publication of annual reports for the High Court, which included the Court reporting against judgment timeliness standards, and the introduction of the Higher Courts Twitter account to improve communications with the public. She has spoken regularly on issues concerning the just and efficient operation of the Courts, and access to justice. This was the topic of her 2011 Ethel Benjamin address, ‘Access to justice, who needs lawyers?’ 13 Otago LR 229. In 2011, following the devastating Canterbury earthquakes, Winkelmann worked with Justice Miller to set up the Earthquake List in Christchurch. The objective of that List was to enable proceedings flowing out of the Christchurch earthquake to be dealt with promptly and in a time frame that met the needs of the community. . Winkelmann also worked with other Heads of Bench to establish Court procedures, including opening and closing of the Court day, so that the Court's processes better reflect the two founding cultures of New Zealand, Māori and Pakeha. Winkelmann was jointly awarded the Australasian Institute of Judicial Administration Award for Excellence in 2013, for her work in judicial administration flowing out of the Christchurch earthquake. Winkelmann joined the Court of Appeal Bench with effect from 1 June 2015. She is the Chief Justice's representative on the Council of Law Reporting. She is also the Chair of the Institute of Judicial Studies, the body responsible for providing continued education to judges. As Chair of the Institute, she oversees a curriculum which supports judges in developing core judicial skills and which also incorporates courses providing judges with the context necessary to enable them to judge in a diverse society.
Rulings
In the high-profile case where US authorities were seeking to have Kim Dotcom arrested and extradited, Winkelmann ruled on 28 June 2012 that the warrants used to seize Dotcom's property were illegal because they were too broad. "These categories of items were defined in such a way that they would inevitably capture within them both relevant and irrelevant material. The police acted on this authorization. The warrants could not authorize seizure of irrelevant material, and are therefore invalid." Winkelmann also ruled that the handing of hard drives seized by New Zealand police in the raid to the FBI was in breach of extradition legislation, and the FBI's removal from New Zealand of cloned data from them was unlawful.
On 17 December 2018, it was announced that Winkelmann would succeed Dame Sian Elias as Chief Justice, and she was sworn in on 14 March 2019, becoming the second woman to hold the office. In the 2019 Special Honours, in preparation for her taking up the role of Chief Justice, Winkelmann was appointed a Dame Grand Companion of the New Zealand Order of Merit.
