1import Icon from 'react-native-vector-icons/MaterialIcons'
2
3Icon.loadFont();
4

1Easy/Working solution 2021. React Native 0.64. XCode 12+
2
3Add these lines in ios/<ProjectName>/Info.plist. Rebuild ios app. that's it!
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5<key>UIAppFonts</key>
6    <array>
7      <string>AntDesign.ttf</string>
8      <string>Entypo.ttf</string>
9      <string>EvilIcons.ttf</string>
10      <string>Feather.ttf</string>
11      <string>FontAwesome.ttf</string>
12      <string>FontAwesome5_Brands.ttf</string>
13      <string>FontAwesome5_Regular.ttf</string>
14      <string>FontAwesome5_Solid.ttf</string>
15      <string>Fontisto.ttf</string>
16      <string>Foundation.ttf</string>
17      <string>Ionicons.ttf</string>
18      <string>MaterialIcons.ttf</string>
19      <string>MaterialCommunityIcons.ttf</string>
20      <string>SimpleLineIcons.ttf</string>
21      <string>Octicons.ttf</string>
22      <string>Zocial.ttf</string>
23    </array>

1On Android it takes the name of the file and you are done. On iOS however it is a bit more complex.
2
3There are a few steps you should take:
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51) Double check the font files are included in the Target in XCode
62) Make sure they are included in the step "Copy Bundle Resources" (add files, not folders)
73) Include them in the Info.plist of the app
84) Find the name of the Font through FontBook or with some log statements in your AppDelegate
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10Explained in more detail here: http://codewithchris.com/common-mistakes-with-adding-custom-fonts-to-your-ios-app/
11
12Good luck!