The Legion of Mary is an association of Catholic laity who serve the Church on a voluntary basis. It was founded in Dublin, Ireland by Frank Duff on 7 September 1921. The main apostolate of the Legion is to support all men and women, young and old, rich and poor including people on the margins of society regardless of religion. Members are primarily engaged in the performance of the Spiritual Works of Mercy rather than works of material aid.

The Saint Peter's Praesidium was formed in the 1950s and was active until at least 1976. It was reformed on 7 March 2007. Meetings are held in the Parish Centre every Tuesday at 11.00 am. The photograph on the right shows members of the Legion of Mary carrying a statue of Our Lady in procession through the Church grounds in May in the late 1950s.

In 2009, the Parish hosted a Peregrinatio Pro Christo - Journeying for Christ. A team of Legionaries from Ireland, shown below, came to Ardrossan on 4 July and were introduced to the congregation during Masses that weekend. Starting on 6 July, they visited as many homes as they could, introducing themselves and briefly inviting people to attend their own Church, return to Church or to take up the habit of prayer. Accommodation was provided by Saint Peter's Parishioners who also served meals daily in the Parish Centre.

On 10 July, there was a special Mass to mark the end of the Peregrinatio Pro Christo. This was followed by a social event in the Parish Centre. The Legionaries returned home on 11 July.

Photographs of the Legion of Mary, including the social event of 10 July 2009, are on the Pictures page.