Mnason, from a Greek name pronounced mnaw-sown, was a highly-regarded Christian from Cyprus who was living in Jerusalem at the time that the apostle Paul returned from his Third Missionary Journey. Mnason is referred to as "old" in the verse quote below in reference to him having been a Christian for a long time (very likely longer than Paul - see Was Paul Among Them?), not necessarily his age; other translations use "early" (RSV). Paul was to lodge at Mnason's house while in Jerusalem:
"21:15 And after those days we took up our carriages, and went up to Jerusalem. 21:16 There went with us also certain of the disciples of Caesarea, and brought with them one Mnason of Cyprus, an old disciple, with whom we should lodge. 21:17 And when we were come to Jerusalem, the brethren received us gladly." (Acts 21:15-17 KJV)
"When we were come to Jerusalem, the brethren received us gladly"
Perhaps the reason that Paul was given to stay with a deeply-rooted Christian who, like Paul, could not be intimidated by anyone was because with Paul was coming trouble - as Paul and the others were given in prophecy before the apostle even got to Jerusalem:
"21:10 And as we tarried there many days, there came down from Judaea a certain prophet, named Agabus. 21:11 And when he was come unto us, he took Paul's girdle, and bound his own hands and feet, and said, Thus saith The Holy Ghost, So shall the Jews at Jerusalem bind the man that owneth this girdle, and shall deliver him into the hands of the Gentiles.
Despite the danger from those who couldn't, or wouldn't yet see, many other Jews of the city did warmly welcome Paul.
"21:17 And when we were come to Jerusalem, the brethren received us gladly. 21:18 And the day following Paul went in with us unto James; and all the elders were present. 21:19 And when he had saluted them, he declared particularly what things God had wrought among the Gentiles by his ministry." (Acts 21:17-19 KJV)
It wasn't long however before the prophecy of Paul's being taken prisoner was fulfilled. Mnason's houseguest enjoyed a very brief stay.
"21:27 And when the seven days were almost ended, the Jews which were of Asia, when they saw him in the temple, stirred up all the people, and laid hands on him, 21:28 Crying out, Men of Israel, help: This is the man, that teacheth all men every where against the people, and the law, and this place: and further brought Greeks also into the temple, and hath polluted this holy place. 21:29 For they had seen before with him in the city Trophimus an Ephesian, whom they supposed that Paul had brought into the temple.
Fact Finder: What civil right did Paul claim when he was arrested by the Jerusalem mob?