Ceredigion MP Mark Williams was one of only two MPs who were not appointed by new leader Tim Farron.
His office said at the time that Mr Williams would be given "a key campaign job" in August.
He has now been appointed vice-chairman of the parliamentary campaigns team and will lead the party's rural communities campaign.
Mr Williams said: "I am delighted to have this new responsibility for rural campaigning across Britain and of course in particular in Wales.
"This brief covers a huge range of issues - the plight of the agricultural industry, the provision of services and infrastructure across rural areas and making sure the needs of rural areas are understood at the very heart of government."
Only Mr Williams and former party leader Nick Clegg - who turned down a role - were not included in the 'cabinet' from the party's eight MPs.
A Welsh Lib Dem MP has been given a new position, just over a week after being left out of the party's 'cabinet'.