Between 1920 and 1946, a total of 63 countries became member states of the League of Nations.
When the Assembly of the League of Nations first met, it consisted of 42 founding members. A further 21 countries joined between then and the dissolution of the League.
Since various nations left the League at different points, its full roster of 63 countries was never represented simultaneously. The League's greatest extent was from 28 September 1934 (when Ecuador joined) to February 1935 (when Paraguay withdrew) with 58 countries. At this time, only Costa Rica (December 1924), Brazil (June 1926), Japan (March 1933), and Germany (October 1933) had withdrawn, and only Egypt joined later (on 26 May 1937).
Founding members
The Covenant of the League of Nations was part of the Treaty of Versailles, signed on 28 June 1919 between the Allies of World War I and Germany. In order for the treaty to enter into force, it had to be deposited at Paris; in order to be deposited, it had to be ratified by Germany and any three of the five Principal Powers (the United States of America<!-- full name deliberately used here to put the Principal Powers in alphabetical order the way the treaty does, with the United States alphabetized under A -->, the British Empire, France, Italy, and the Empire of Japan). Any Allied signatory that ratified the treaty would automatically join the League.
Germany was forced to ratify the treaty first, which it did on 10 July 1919. Italy ratified on 7 October 1919, the British Empire and its colonies ratified on 10 October 1919, and France ratified on 12 October 1919. These ratifications were deposited (along with those of ten other countries) on 10 January 1920. All but three of the remaining signatories (the United States, Ecuador, and the Kingdom of Hejaz) had ratified by the time the Assembly first met on 15 November 1920.
The Covenant also invited 13 neutral nations to join the League. To join, an invitee had to agree ("accede without reservation") to the Covenant within two months of the League's founding. All 13 invitees accepted their invitations within the two-month deadline; four of them accepted their invitations even before the League was founded.
The United States Senate voted in favor of ratification, but this failed to reach the necessary two-thirds majority. Hejaz refused to join the League due to the League giving control of Palestine from the broken-up Ottoman Empire to the British Empire. Ecuador would not ratify the treaty until 1934; as an original Allied signatory, it was entitled to join the League this way with no deadline or Assembly vote, and the League considered Ecuador to still be a founding member.
Uniquely, China (then the Beiyang government) joined the League by ratifying the Treaty of St. Germain (with Austria) instead of the Treaty of Versailles (with Germany), as both treaties included the Covenant of the League of Nations. It had refused to sign the Treaty of Versailles due to it giving the previously-German-controlled Shandong colony in China to the Empire of Japan.
Additional members could join the League by a two-thirds vote of the Assembly.
List
Founding members below are listed in the order matching the Covenant (signatories first, followed by invitees), except for Ecuador. The colonies of the British Empire immediately followed it and did not sign in alphabetical order. Czechoslovakia and El Salvador were alphabetized under 'S'.
<!-- Editor's note: "Invitee" is explicitly added to all of the invitees for accessibility. Signatories can then be distinguished because they joined by November 1920 except for Ecuador. This setup adds the least amount of extra repeated text or columns. -->
A country was allowed to send a withdrawal notice at any time, but it would take two years to come into effect after the League received it, and meanwhile the country would retain both its obligations and its membership. In practice, a withdrawing country quit the League immediately, and was absent from all sessions and votes for the last two years of its formal membership. The "Date of withdrawal" column shows when the withdrawal notice was sent.
{| class="wikitable sortable plainrowheaders"
|+ List of members
! scope="col" | Member state
! scope="col" | Date of admission
! scope="col" | Date of withdrawal
! scope="col" class=unsortable | Notes
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| Withdrew upon rejection of a resolution to give Brazil the United States' vacant permanent Council seat.
|-style="background:#cdf;"
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| rowspan="6" |
| rowspan="6" |
| rowspan="6" | All six members joined from one ratification document.
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| scope="row" data-sort-value=Africa |