It is best to serve the adverse party first because the County Clerk will often require that proof of service on the adverse party be attached to the notice of appeal filed with that office.
It may happen, however, that the party appealing will do only one of the two steps. For example, he may serve the adverse party but forget to file the notice of appeal with the County Clerk.
When this occurs the party appealing can utilize CPLR § 5520(a) to secure permission from the trial or appellate court for an extension of time to cure the omission. Authority to grant permission to cure the omission exists under the statute as long as the initial service or filing was "timely." Timeliness is established by CPLR § 5513. In sum, if one of the two steps is done on time, a court can grant permission for a late step two.
All of this is critical because the timely taking of an appeal is jurisdictional: an appellate court has no jurisdiction to entertain an appeal if the notice of appeal is served and filed late.
The Court of Appeals concluded today in M Entertainment, Inc. v. Leydier that the Appellate Division, First Department, erroneously dismissed an appeal when the appellant timely filed the notice of appeal with the County Clerk, but did not employ the correct method of service by mail on the adverse party. Instead of dismissing the appeal, the Appellate Division should have determined whether to exercise its discretion under CPLR § 5520(a) to grant additional time to serve the adverse party in the correct manner.
The case is a good reminder of the importance of serving and filing the notice of appeal in a timely fashion, and it is also a good reminder of the availability of CPLR § 5520(a) when either the filing or the service is not done in a timely manner. The decision can be found here.