Cleanaway was found guilty of constructive dismissal and unlawful harassment on grounds of sexual orientation at the Employment Tribunals in London this week.
Business transformation manager Robert Whitfield, 28, was subjected to "persistent" discrimination that "seriously damaged" his employment relationship, said the panel.
The tribunal's judgement goes to the core of the company, with senior managers and its human resources department blamed.
The panel heard that Whitfield never told his bosses he was gay as he considered it to be a personal matter.
Cleanaway's actions received widespread media coverage as it was a landmark ruling - and included a reference to hit TV show Little Britain.
Whitfield was reportedly the first person to win a case under the new Employment Equality (Sexual Orientation) Act.
In the tribunal he described how he was frequently called Sebastian by his colleagues, a nickname he felt referred to the camp Prime Minister's assistant in the BBC comedy.
Sales and marketing director Paul Jackson and UK sales manager Martin Hadfield were accused of humiliating Whitfield in front of his colleagues because he was gay, while Cleanaway's human resources department was understaffed, acted inappropriately at times and gave "muddled evidence" to the court, according to the judgement.
Whitfield suffered a five-month period of harassment at the firm's Brentwood, Essex offices, ending when he could take no more and handed his notice in on May 6 last year, the judgement found.
Among other incidents, he was told to wear a t-shirt with pink lettering on it, called a queen and saw a presentation he prepared for a sales meeting sabotaged.