The appointment completes a three-month recruitment process, begun when Yvonne Iwaniuk quit in August (PRWeek, 6 September).
Sexton-Chadwick takes up the role in February, when her Diageo notice period expires.
She will take charge of all brand PR and corp comms in the UK and Ireland, reporting to UK and Ireland marketing director Rod Connors. She will handle issues and crisis management and CSR.
Nike has retained Freud Communications and Radiator PR for consumer PR work for two-and-a-half years.
In addition, Nike also uses Karol Marketing for consumer work on its 'all-conditions' gear and Renaissance PR for work on its golf clothes and equipment.
Both UK PR manager Charlie Brooks and UK corporate communications manager Simon Charlesworth will report to Sexton-Chadwick.
Brooks said no review of PR strategy or agency relationships was planned.