Actually it was Lincoln's Secretary of State. Read your history and you will find that Johnson hands were tied by the Tenure of Office Act. When he tried to fire his Secretary of War, Stanton, he was impeached. The fact that there was no VP and the next in line was totally unacceptable to a majority of Senators led to him surviving by a single vote.
Alaska was purchased when Johnson was President but Seward was definitely Abe's guy.