Sir William launched an inquiry in June, after Mr Kogan revealed during a confirmation hearing with MPs in May that he had contributed money to Nandy’s 2020 leadership campaign, as well as that of the prime minister.
His probe was into whether the appointment process breached the code that governs how people are appointed into public roles.
In his report, external, Sir William said Mr Kogan had made two donations of £1,450 each to Nandy’s 2020 campaign, one personally and one through his company.
They were part of total donations worth £33,410 to Labour and the party’s candidates in the five years prior to his appointment, the commissioner said.
The two donations to Nandy were below declaration thresholds set by the Electoral Commission and by Parliament.
But Sir William found that Nandy was “readily able to check” whether Mr Kogan had donated to her campaign before announcing him as her pick for chair, and should have done so prior to his appointment.
“It need not be true that the donations actually influenced the secretary of state’s decision-making – only that the risk of this perception should have been mitigated by declaration of this financial interest,” he wrote.
“The commissioner finds that she has unknowingly breached the code. She should have carried out that check and taken any necessary consequential action before selecting him,” he added.