Mehta challenged his sentence in the Court of Appeal but this was declined, with the court finding no error in the sentencing judge's methodology, according to a statement by the Commerce Commission.
The commission welcomed the decision.
"We are pleased that the courts have confirmed that imprisonment was appropriate. This aligns with our view of the seriousness of Mr Mehta's conduct," said commissioner Anna Rawlings.
"Traders need to know that there can be serious personal consequences for offending against consumers."
Mehta used Flexi Buy's income for his personal use including rent on his Auckland apartment, living expenses and at least $22,000 spent during a 2013 trip to India, the commission said.
The company was last year fined $50,000 after being convicted under the Credit Contracts and Consumer Finance Acts for describing some key information in a misleading or deceptive way.
Other charges included failing to provide customers with adequate disclosure of key information about their credit contracts such as the frequency or amount of payments, default interest and the debtors' cancellation rights.
Customers who purchased goods but never received them were awarded almost $4000 in damages.
The case against Flexi Buy and Mehta was the result of a year-long Commerce Commission investigation into the industry that found 31 of the 32 mobile traders it had identified did not comply with trading laws.