In 1984, Penberthy was made a deaconess in the Church of England. From 1984 to 1985, she served as a full-time deaconess at St Andrew's Church, Haughton-le-Skerne in the Diocese of Durham. She moved to Wales in 1985, and began her ministry in the Church in Wales. From 1985 to 1989, served as a full-time deaconess in the Benefice of Llanishen and Lisvane on the outskirts of Cardiff and in the Diocese of Llandaff. In 1987, Penberthy was ordained a deacon; the Church in Wales had ordained women to the diaconate since 1980. From 1987 to 1989, she was also a non-stipendiary minister in the benefice of Llanishen and Lisvane. From 1989 to 1993, she was a non-stipendiary minister in the benefice of Llanwddyn and Llanfihangel-yng-Nghwynfa and Llwydiarth in the Diocese of St Asaph; her husband served as its Vicar during this period. From 1993 to 1995, she was a non-stipendiary minister in the benefice of St Sadwrn's Church, Llansadwrn with Llanwrda and Manordeilo in the Diocese of St Davids; her husband was vicar of this benefice from 1993 to 2010. From 1994, she was also a Provincial Officer in the Division for Parochial Development and Renewal; this was an appointment focused on evangelism. In 1997, Penberthy was ordained a priest; this was the first year that the Church in Wales ordained women to the priesthood, making her one of the first female priests in the province. She continued working for the Division for Parochial Development and Renewal until 1999. From 1999 to 2001, she was Priest-in-Charge of St Cynwyl's Church, Cynwyl Gaeo with Llansawel and Talley in the Diocese of St Davids. She was made Vicar of the benefice in 2001. She was also an Adult Education Officer for the Diocese of St Davids between 2001 and 2002, and Warden of Readers for the diocese between 2002 and 2010. In February 2007, she was also made a Canon of St Davids Cathedral; she was the first woman to be appointed a canon at that cathedral. In 2010, Penberthy returned to England, leaving behind her benefice and canonry. From 2010 to 2011, she was Priest-in-Charge of the benefice of Charlton Musgrove , Cucklington and Stoke Trister in the Diocese of Bath and Wells. She was made Rector of the benefice in 2011. In July 2015, it was announced that she would be leaving the diocese to return to Wales. On 8 September 2015, she was inducted as the Rector of Glan Ithon in the Diocese of Swansea and Brecon.
Episcopal ministry
On 2 November 2016, it was announced that Penberthy had been elected the next Bishop of St Davids. She is the first woman to be elected a bishop in the Church in Wales. Her election was confirmed on 30 November 2016, thereby legally becoming the 129th Bishop of St Davids. She was consecrated a bishop on 21 January 2017 during a service at Llandaff Cathedral, and she was enthroned at St Davids Cathedral on 11 February 2017.
Personal life
Penberthy is married to Adrian Legg who is also an Anglican priest. Together, they have four children.