I had the same issue with some mstest integration tests I created.
I would recommend using the DeploymentItem attribute on your test class instead of a direct reference in code. For example, this is what I have:
[TestClass]
[DeploymentItem("Unity.config")]
[DeploymentItem("EntityFramework.SqlServer.dll")]
**[DeploymentItem("AutoMapper.Net4.dll")]**
public class MyFancyIntegrationTests
{
You also need to add a reference, as mentioned above, using nuget, to automapper in your test project.
To troubleshoot this, view the bin directory of your mstest project. You should see both Automapper.dll and AutoMapper.Net4.dll if you have automapper referenced in the project. Prior to adding the DemploymentItem attribute to your test class, You won't see AutoMapper.Net4.dll in your TestResults[MyLatestTestRun] folder. But add the attribute and you will.
MSTest does not play well with injection...