The focus of this paper is to investigate the problem of formation control for linear multi-agent systems through output feedback, utilizing the asynchronous sampled-data mechanism. Each agent matches one state observer to estimate its own state by only using the asynchronously sampled outputs. Based on the asynchronously sampled estimates of states, we develop a formation control law that operates in a distributed manner. The establishment of auxiliary time sequences plays a significant role in coping with the difficulty of asynchronous sampling. Through the study, an explicit upper bound of sampling periods and admissible formation condition, which guarantee that the predefined geometric shape is asymptotically formed, are given. Two examples are eventually carried out to show that formation is achieved successfully.