The virion of Staphylococcus phage 812 is formed by an icosahedral capsid and a contractile tail joined together by neck proteins. Despite the role of the neck proteins in virion assembly, DNA packaging, and regulation of genome release, their functions are not well characterized. Here we show that the neck of phage 812 consists of portal, adaptor, stopper, and tail terminator proteins decorated on the outside by two types of cement proteins. A dodecameric DNA-binding site on the portal complex anchors the genomic terminus inside the capsid, which may prevent an accidental escape of the DNA during the initial stages of genome packaging. The adaptor complex induces a local B-to-A form transition of the DNA in the neck channel that may serve to pause genome translocation. The gating loops of the stopper proteins prevent genome loss from fully packaged proheads by blocking the neck channel prior to the tail attachment. The binding of the tail terminator complex to the stopper complex induces opening of the gating loops and advancement of DNA into the tail. The structure of neck proteins is unchanged by tail sheath contraction. The expulsion of the tail tape measure protein rather than tail sheath rearrangement thus triggers genome release. Our results explain how the active interplay between neck proteins and the genome directs DNA packaging, prevents premature genome release, and enables its ejection into the host cell.