Ymunodd Frances â’r Gwasanaeth Sifil yn Arolygydd Trethi dan hyfforddiant, gan arbenigo ym maes treth gorfforaeth busnesau mawr a’r sector yswiriant bywyd. Pan gafodd Senedd newydd yr Alban ei sefydlu, gweithiai Frances ar weithredu pwerau treth amrywiadwy yr Alban. Symudodd wedyn i Weithrediaeth yr Alban, lle bu’n gweithio ar bolisi tai, cyn symud i faes polisi trafnidiaeth.
Frances joined the Civil Service as a trainee Inspector of Taxes, specialising in large business corporation tax and the life insurance sector. When the new Scottish Parliament was set up, Frances worked on the implementation of the Scottish Variable Tax powers and then moved to the Scottish Executive, where she worked on housing policy before moving to transport policy.