Expositor's Bible Commentary Commentary Mark 16:20

"And they went forth, and preached everywhere, the Lord working with them, and confirming the word by the signs that followed. Amen." — Mark 16:20 (ASV)

“After the Lord Jesus had spoken to them” may refer to vv.15–18 or to some other occasion. The Ascension had been predicted by Jesus (cf. 14:7) and was witnessed by the apostles (cf. Acts 1:9); his sitting at God’s right hand was a matter of faith but firmly believed and preached in the early church (cf. Acts 2:33– 35; 7:56).

There is nothing like v.20 in any of the Gospels. It sounds more like a summary statement from the book of Acts of the activities of the apostles.