Did the atonement end on the Cross? John 19:30  When Jesus therefore had received the vinegar, he said, It is finished: and he bowed his head, and gave up the ghost. 


When Jesus said it is finished did he mean that the atonement for sin had ended? if so what about His ministry as a High Priest what happens to it?
That is certainly how that is interpreted by a large percentage of Christian groups, and it's how the author of Hebrews interpreted it.


  Hebrews 10:10 KJV By the which will we are sanctified through the
  offering of the body of Jesus Christ once for all.


I assume the second part of the question deals with the fact the high priest offers sacrifices for sins. In Hebrews 5:1-3 we read...


  Hebrews 5:1-3 New International Version (NIV)
  
  5 Every high priest is selected from among the people and is appointed
  to represent the people in matters related to God, to offer gifts and
  sacrifices for sins. 2 He is able to deal gently with those who are
  ignorant and are going astray, since he himself is subject to
  weakness. 3 This is why he has to offer sacrifices for his own sins,
  as well as for the sins of the people.


Also


  Hebrews 10 1-18 NIV 
  
  Christ’s Sacrifice Once for All 
  
  10 The law is only a shadow of the
  good things that are coming—not the realities
  themselves. For this reason it can never, by the same sacrifices
  repeated endlessly year after year, make perfect those who draw near
  to worship. 2 Otherwise, would they not have stopped being offered?
  For the worshipers would have been cleansed once for all, and would no
  longer have felt guilty for their sins. 3 But those sacrifices are an
  annual reminder of sins. 4 It is impossible for the blood of bulls and
  goats to take away sins. 5 Therefore, when Christ came into the world,
  he said: “Sacrifice and offering you did not desire,
      but a body you prepared for me; 6 with burnt offerings and sin offerings
      you were not pleased. 7 Then I said, ‘Here I am—it is written about me in the scroll—
      I have come to do your will, my God.’”[a] 8 First he said, “Sacrifices and offerings, burnt offerings and sin offerings you did
  not desire, nor were you pleased with them”—though they were offered
  in accordance with the law. 9 Then he said, “Here I am, I have come to
  do your will.” He sets aside the first to establish the second. 10 And
  by that will, we have been made holy through the sacrifice of the body
  of Jesus Christ once for all. 11 Day after day every priest stands and
  performs his religious duties; again and again he offers the same
  sacrifices, which can never take away sins. 12 But when this priest
  had offered for all time one sacrifice for sins, he sat down at the
  right hand of God, 13 and since that time he waits for his enemies to
  be made his footstool. 14 For by one sacrifice he has made perfect
  forever those who are being made holy. 15 The Holy Spirit also
  testifies to us about this. First he says: 16 “This is the covenant I
  will make with them
      after that time, says the Lord. I will put my laws in their hearts,
      and I will write them on their minds.”[b] 17 Then he adds: “Their sins and lawless acts
      I will remember no more.”[c] 18 And where these have been forgiven, sacrifice for sin is no longer necessary.


This is referring to a human priest, who must sacrifice "for his own sins, as well as for the sins of the people". Jesus, as the sinless Son of God, was sinless, so He would not have to sacrifice for His own sins.

This means that these sacrifices are ordinary sacrifices offered by sinful humans. Offering animals - the best animals, spotless, presumably, if they were following the Law, but animals nonetheless.

Jesus' sacrifice was unique.  It was offered by a preist who was sinless (Christ) and the offering was perfect - something unlike any other offering that ever had, or could, be offered (The sinless Son of God).

So His sacrifice only needed to be a one-time sacrifice.

And the Priestly title?  His work as Priest, at least as far as sacrifice is concerned, is finished, but He still retains the title, because it describes an aspect of who He is. The author of the passage above was showing that Jesus was greater than any other High Priest.

Much more can be found here: http://www.mycrandall.ca/courses/NTIntro/Melch8.htm

Excerpt:


  Finally, in Heb 7:18, the author argues that, if there is a new High
  Priest, there must be a setting aside (athetêtis) of the (old)
  commandment (regarding priesthood) because it was weak and useless
  (asthenês kai anôpheles) (see "fleshly commandment" in Heb 7:16). As
  he already indicated, a new priesthood would not have neen necessary
  if the old priesthood was effective. So, according to the author, the
  promise of a perpetual Levitical priesthood in Exod 29:9 ("And they
  shall have the priesthood by a perpetual statute") and 40:14 ("Their
  anointing will qualify them for a perpetual priesthood throughout
  their generations") should be interpreted to mean until the time of
  the eschaton, when the greater High Priest appears. The author
  denigrates the Law further by saying that it made nothing “perfect”
  (eteleiosen) (Heb 7:19a). Actually, according to the author, the Law
  was not intended to be eternal but only preparatory for something
  better and perfect. When he says that the Law did not make perfect, he
  means that the Law as pertaining to the priesthood and the Temple cult
  did not serve to make anyone acceptable to God. As a result a better
  hope is introduced (through a better High Priest), which allows for a
  new approach to God (7:19b).