Use

git show --summary `git merge-base foo master`

If you’d rather see it in context using gitk, then use

gitk --all --select-commit=`git merge-base foo master`

(where foo is the name of the branch you are looking for.)

First, if you branch was created within gc.reflogexpire days (default 90 days, i.e. around 3 months), you can use git log -g <branch> or git reflog show <branch> to find first entry in reflog, which would be creation event, and looks something like below (for git log -g):

Reflog: <branch>@{<nn>} (C R Eator <creator@example.com>)
Reflog message: branch: Created from <some other branch>

You would get who created a branch, how many operations ago, and from which branch (well, it might be just "Created from HEAD", which doesn't help much).

That is what MikeSep said in his answer.

Second, if you have branch for longer than gc.reflogexpire and you have run git gc (or it was run automatically), you would have to find common ancestor with the branch it was created from. Take a look at config file, perhaps there is branch.<branchname>.merge entry, which would tell you what branch this one is based on.

If you know that the branch in question was created off master branch (forking from master branch), for example, you can use the following command to see common ancestor:

git show $(git merge-base <branch> master)

You can also try git show-branch <branch> master, as an alternative.

This is what gbacon said in his response.

I'm not sure of the git command for it yet, but I think you can find them in the reflogs.

.git/logs/refs/heads/<yourbranch>

My files appear to have a unix timestamp in them.

Update: There appears to be an option to use the reflog history instead of the commit history when printing the logs:

git log -g

You can follow this log as well, back to when you created the branch. git log is showing the date of the commit, though, not the date when you made the action that made an entry in the reflog. I haven't found that yet except by looking in the actual reflog in the path above.

This did it for me: (10 years later)

git log [--remotes] --no-walk --decorate

Since there is no stored information on branch creation times, what this does is display the first commit of each branch (--no-walk), which includes the date of the commit. Use --remotes for the remote branches, or omit it for local branches.

Since I do at least one commit in a branch before creating another one, this permitted me trace back a few months of branch creations (and feature dev-start) for documentation purposes.

source: AnoE on stackexchange

This commands shows the created date of branch dev from main

$git reflog show --date=iso dev
$7a2b33d dev@{2012-11-23 13:20:28 -2100}: branch: Created from main

How to know via Github GUI

I show all the answers and no one gave an answer with UI. If anyone likes to see when the branch is created via Github UI.

1. Go to Insights Tab.
2. Select the Network tab in the sidebar.

You can see the branch creation like as below

I found the best way: I always check the latest branch created by this way

git for-each-ref --sort=-committerdate refs/heads/

syntax: git reflog --date=local | grep checkout: | grep ${current_branch} | tail -1

example: git reflog --date=local | grep checkout: | grep dev-2.19.0 | tail -1

result: cc7a3a8ec HEAD@{Wed Apr 29 14:58:50 2020}: checkout: moving from dev-2.18.0 to dev-2.19.0

Combined with the answer from Andrew Sohn (https://stackoverflow.com/a/14265207/1929406)

branchcreated=$(git reflog show --date=format:'%Y-%m-%d %H:%M:%S' --all | sed 's!^.*refs/!refs/!' | grep '/master' | tail -1| cut -d'{' -f 2| cut -d'}' -f 1 | xargs)
echo $branchcreated

If you want to get the details for all the branches

for i in `git branch -r | tail -n +2 `;do git log --reverse $i|grep -A 2 -B 2 `echo $i | awk -F'origin/' '{print $2}'` |head -n 4; done