In February, 2018, Mangau announced an inititative to destroy rogue crocodiles observed near Bonegi, Guadacanal, and assured the public that managing the crocodile population was a priority for the Police.
He was promoted to Deputy Commissioner of Police from Assistant Commissioner of Police, on October 24, 2019. He was also appointed to the Royal Solomon Islands Police Force Executive. His responsibilities include National Security.
- "RSIPF announces new members of executive team". Solomon Islands World News. 2019-10-25. https://theworldnews.net/sb-news/rsipf-announces-new-members-of-executive-team. Retrieved 2019-11-15. "The officers are Mr Mostyn Mangau who is promoted to the rank of Deputy Commissioner, and Mr Ian Bara and Ms Evelyn Thugea who are promoted to the rank of Assistant Commissioner."
- Caroline Tiriman (2018-09-18). "Solomon Islands Ileksan securiti" (in Tok Pisin). Australian Broadcasting Corporation. https://www.abc.net.au/tok-pisin/2018-09-18/solomon-islands-ileksan-securiti/10264688. Retrieved 2019-11-15. "Despla registration bai pinis long namba 27 long despla mun. Nau iet tu ol polis iwok long mekim ol treining blong helpim ol i mekim gut wok blong ol long lukautim ileksan, Assistant Commissioner National Operations, Mostyn Mangau itok oli go hed nau long wok wantem ol narapla laen olsem electoral komisin, wantem tu ol NGO long redi-im ol pipal long ileksan."
- "RSIPF OFFICERS RECOGNISED IN 2018 QUEEN’S BIRTHDAY HONOURS AND AWARDS". Royal Solomon Island Police Force. 2018. http://www.rsipf.gov.sb/?q=node/578. Retrieved 2019-11-15. "Three of the police officers were awarded the British Empire Medal (BEM). The officers are Assistant Commissioner, Mostyn Mangau; Chief Superintendent Ian Bara and Chief Superintendent Stanley Riolo."
- "Police launch operation to destroy crocodile sighted in Bonegi". Solomon Islands Broadcasting Corporation. 2018-02-09. https://www.sibconline.com.sb/police-launch-operation-to-destroy-crocodile-sighted-in-bonegi/. Retrieved 2019-11-16. "Assistant Police Commission, National Operations Mostyn Mangau said managing the crocodile population is a priority for the Police."