After denunciation by a clergyman, Horobin was held on indecency charges in May 1962, accused of several charges of indecent assault on boys aged under 16 in 1958 and 1959. A 17-year-old boy was also held with Horobin and he was jointly charged with committing two acts of indecency with Horobin in 1961, and attempting to procure another man for an act of indecency with Horobin.

As Warden of Mansfield House University Settlement, Horobin lived in a small flat there. According to Michael Bloch, Horobin's homosexual tastes were common knowledge among the residents, who were rewarded for yielding to Horobin's demands. He apparently formed "affectionate relationships" with some of them.

At the trial in June 1962, where Horobin was defended by Peter Rawlinson, QC, the prosecution alleged that Horobin had told his secretary that he was "virtually married" to the 17-year-old. Several witnesses at the trial testified to Horobin's behaviour and the manner of his assaults.

Horobin admitted ten charges of assault in July 1962, and was sentenced to four years imprisonment. Horobin had told the secretary of the settlement that his relationships with boys had been going on for over 40 years and described homosexuals as "... us poor devils who are born like this; nothing can change me. It is natural for us to love boys in this way."

Unrepentant, Horobin told his friend John Betjeman: "I broke the law with me eyes open all my life until I went to prison. I broke it in prison. I broke it immediately I came out of prison, and I have not the slightest intention of ever paying any attention to it."

Horobin moved to Tangier, Morocco, after his release from prison, and died there in 1976.