Catriona MacLennan in 1999 was involved in setting up Nga Ture Kaitiaki ki Waikato Community Law Centre. The Centre opened on 1 February 2000 in Otahuhu. There were two staff - Catriona and a community worker, Carol Tomuri Te Awa. The Centre dealt with a wide range of legal issues, including employment, immigration, family, criminal, traffic, consumer, credit and benefit laws. The Centre also rapidly increased in size, with a number of lawyers, volunteers and others coming on board. Catriona returned to work at the Centre in 2005, by which time it had moved to Manukau after the District Court shifted from Otahuhu to Manukau.
In 2010, Catriona was the Project Director for Nga Tangata Microfinance Trust, a loan scheme set up to provide loans at no interest to families on low incomes so they would not be forced to borrow from lenders charging very high interest rates and fees. The scheme was established by Child Poverty Action Group, the Federation of Family Budgeting Services, the New Zealand Council of Christian Social Services and Good Shepherd.
Catriona in 2013 wrote the Animal Agenda Aotearoa, a 10-point plan to improve animal welfare in New Zealand. Catriona wrote two features in Animals' Voice magazine to explain the Agenda. The Agenda was used to lobby political parties to improve their animal welfare policies in the run-up to the 2014 general election.
Catriona in 2014 wrote the Women's Election Agenda 2014, a 100-point plan to improve the position of women in New Zealand. The Agenda was sent to all political parties in the run-up to the 2014-general election and was used to lobby parties to improve their policies on issues such as domestic violence, the gender pay gap and rape law reform.