Mr Mackay resigned from the Scottish Government after admitting he "behaved foolishly" by sending unsolicited messages to a 16-year-old boy on social media.
He was suspended from the SNP and now sits as independent MSP for Renfrewshire North and West, but has not taken part in any debates or votes since his resignation.
However, expenses records show he claimed for accommodation in Edinburgh for 10 days in July, according to the BBC.
The records for the period immediately after Mr Mackay quit are yet to be published by the parliament, but BBC Scotland said it had seen claims filed by Mr Mackay for rent totalling £327.10 in early July.
The Scottish Parliament was in recess at the time, although MSPs did sit on one of the 10 days involved for First Minister Nicola Sturgeon to update them on the coronavirus pandemic.
Since he remains an MSP, Mr Mackay is entitled to claim under Holyrood’s Edinburgh accommodation provision and there is no rule that he must attend parliament to do so.
Hours before Mr Mackay was due to present the Scottish Government’s budget to the parliament in February, the Scottish Sun newspaper revealed he had sent hundreds of messages to a teenage boy over a six-month period.
He issued a statement apologising "unreservedly" to the boy and resigned from the Cabinet.
Police later concluded there was "nothing to suggest that an offence has been committed". The SNP suggested in March that Mr Mackay was "under medical supervision".
Scottish Conservative MSP Graham Simpson has previously proposed that MSPS who do not attend Holyrood for six months should be forced to stand down.
Today Mr Simpson said: “This is just the latest example of the shamed former Finance Secretary continuing to treat voters with contempt.
“His constituents will be appalled that he was claiming money for accommodation in Edinburgh despite not turning up to parliament and working for them.
“It shows exactly why we need my Mackay’s Law to be put into legislation in order to oust MSP’s like him who shun their responsibilities to the public.”
A spokesman for Mr Mackay said the expenditure "complies with Scottish Parliament allowance rules" and "covered the notice period and requirements of terminating the accommodation tenancy".
He added: "Mr Mackay's Renfrewshire North and West constituency office is continuing to operate remotely, in line with the Scottish government's coronavirus guidance, in dealing with casework and making representations on behalf of constituents.".