The hearing before Derbyshire social services director Bruce Buckley decided to issue Peter Moore with a final written warning.
The director ruled that Mr Moore, whose salary is paid by the department while he carries out fulltime union duties, had committed 'a serious breach of trust' by being involved in unlawful industrial action.
The council says Mr Moore denied any involvement in a walkout of around 100 social services staff on 9 October, which took place against official Unison advice.
'The council has made an enormous investment in proper trade union representation. We greatly value the involvement and active participation in proper trade union representation. We greatly value the involvement and active participation of trade unions in the workplace. I have no doubt the services we provide to the people of Derbyshire would be poorer without their contribution.'
Mr Moore was suspended on full pay almost two weeks ago.